The present invention, in some embodiments thereof, relates to chemistry and, more particularly, but not exclusively, to novel processes of tacticity-controlled olefin polymerization, to catalyst systems which comprise as a pre-catalyst novel complexes of Group IV metals which can be utilized in these processes, and to novel ligand precursors for preparing the pre-catalyst novel complexes.
The huge plastics industry produces a broad variety of polymeric materials having a broad range of properties. These plastic materials are derived from a small group of building blocks—monomers—including ethylene and propylene. The properties of the polymeric materials depend on the nature of these building blocks and on the process employed to assemble these building block. Most of these processes rely on catalytic polymerization.
The nature of the catalyst has a crucial role in determining the microstructure of the polymer, thus determining the physical properties of the resulting plastic. Molecular weight, molecular weight distribution, and above all, the type and degree of stereoregularity (tacticity) and regioregularity (head-to-tail enchainment) affect the properties of the resulting polymer. For example, three familiar forms of polypropylene are: isotactic, in which all methyl side groups are pointing in the same direction in the stretched chain; syndiotactic, in which the methyl side groups point at opposite directions alternatingly; and atactic, in which the methyl groups are pointing randomly in the two directions. A higher degree of stereoregularity (and regioregularity) leads to a better-defined polymer.
For example, isotactic polypropylene (iPP) is a thermoplastic material of vast importance and an ever-increasing demand derived from its useful physical properties and the availability of its feedstock—propylene. The most important microstructural property of polypropylene is the degree of isotacticity which, combined with sufficiently high molecular weight, determines its melting point (Tm) and thereby its possible applications. In an example, polypropylene having a very high degree of isotacticity has a melting transition of Tm=165° C. whereas an atactic polypropylene is a viscous oil.
The type and degree of tacticity are determined by the catalyst employed. Other properties determined by the catalyst include the polymer chain-lengths and chain-length distributions, backbone rearrangement, regio-regularity, ability to incorporate different monomers, etc. Successful catalysts need to be sufficiently active under industrially-relevant conditions.
Most of the industrial catalytic processes employed in ethylene and propylene polymerizations and copolymerizations rely on heterogeneous catalysis processes, and most of which, on heterogeneous Ziegler-Natta type catalysts. Ziegler-Natta catalysts, which are Group IV-metal compounds (and in particular titanium chloride adsorbed on magnesium chloride) activated with alkyl-aluminum co-catalysts, were invented in the 1950's. Ziegler-Natta catalysts of the current generation are highly active and enable the production of highly isotactic polypropylene (having a melting point of 165° C.). Yet, their heterogeneous nature leads to a broad molecular weight distribution (PDI=Mw/Mn>3.5), and to considerably lower activities towards higher olefins.
Homogeneous catalysts for olefin polymerization were developed in parallel. Most of these systems are based on Group IV transition metals (Ti, Zr, Hf) and feature cyclopentadienyl-type (Cp-type) rings as spectator ligands (groups that do not detach from the metal during the catalytic process). Systems that include two Cp-type rings are generally referred to as metallocenes, and systems that include a single Cp-type ring are referred to as half-metallocenes. Using Cp-type containing systems requires different co-catalysts for their activation, which include MAO (methyl aluminoxane) or various boron-based activators (often combined with aluminum based quenchers). When MAO is employed as a co-catalyst, it is usually taken in large excess relative to the pre-catalyst, with a typical ratio ranging from 1000:1 to 10000:1 MAO:pre-catalyst.
Metallocenes were investigated very intensively during the past three decades and have been the subject of numerous publications describing various structural modifications and their applications in propylene and other olefin polymerizations. Correlations between the symmetries of the catalysts and the tacticities of the resulting polymers were established (Ewen Rules). Yet, the commercial applications of the metallocenes are limited due to their high cost and oftentimes by an inferior isotacticity obtained for the resulting polypropylene.
In the past 15 years, there has been a worldwide interest in development of “cyclopentadienyl-free systems”—homogeneous pre-catalysts devoid of a cyclopentadienyl ring. This interest was driven by the over-crowdedness of the metallocene area, and by the realization that modified catalysts leading to polymers of new or improved properties could be developed. These non-metallocene systems include variable transition metals, and still, the most promising systems in terms of activities and stereospecificities are based on the Group IV transition metals. Some of these catalysts have shown remarkable activities, including living polymerization of high olefins at room temperature, highly active polymerization of ethylene, and the combination of living and isospecific polymerization of high olefins.
Octahedral complexes of Group IV metals have proven to be valuable catalysts for stereoregular olefin polymerization [Lamberti et al., C. Coord. Chem. Rev. 2009, 253, 2082]. In particular, C2-symmetric catalysts of “sequential” tetradentate-dianionic ligands featuring the {ONNO}, {OOOO}, and {OSSO} cores were found to lead to isoselective polymerization of high-olefins and propylene [see, for example, Tshuva et al. J. Am. Chem. Soc. 2000, 122, 10706; Segal et al. Organometallics, 2005, 24, 200; Gendler et al. J. Am. Chem. Soc. 2008, 130, 2144; Cohen et al. Macromolecules, 2010, 43, 1689; Busico et al. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15321; U.S. Pat. No. 7,241,714; Kiesewetter et al. J. Am. Chem. Soc. 2010, 132, 5566; Cohen et al. Inorg. Chem. 2007, 46, 8114; Ishii et al. J. Am. Chem. Soc. 2009, 131, 13566; and Capacchione et al. J. Am. Chem. Soc. 2003, 125, 4964].
Yet, the structural diversity of symmetric ligands is limited. The much broader variety of non-symmetric ligands should yield C1-symmetric polymerization catalysts of superior performance.
For example, a family of catalysts recently developed by the Symyx company and found commercial application by the Dow company includes C1-symmetric hafnium complexes having pyridyl-amido-type ligands (Boussie et al. Angew. Chem. Int. Ed. 2006, 45, 3278.). These catalysts polymerized propylene to a high-molecular weight polypropylene. Notably, the highest melting point described for these polymers was Tm of approximately 150° C.
Thus, except for scarce cases, the tacticity induction in propylene polymerization by non-metallocenes is inferior in comparison to the best metallocenes and to the latest generation of heterogeneous Ziegler-Natta catalysts.
Salalen ligands are “sequential” tetradentate-dianionic ligands that include a neutral imine-donor, a neutral amine-donor, and two anionic phenolate groups. Salalen ligands may be regarded as half-Salan/half-Salen hybrid ligands. The coordination behavior of Salalen ligands was found to reflect that of its symmetric predecessors.
A preliminary report described a Salalen ligand featuring tert-butyl substituents on the two phenolate rings. This Salalen ligand was found to wrap around octahedral Group IV metal centers diastreoselectively so that the half-Salan O—N—N donors bound in a fac-mode and the half-Salen O—N—N donors bound in a mer-mode, yielding C1-symmetric complexes with cis-related labile groups. The two labile groups experience different steric and electronic influence, as one of them is trans to the imine neutral N-donor while the other is trans to the phenolate ring O-donor (being proximal to the amine donor). Complexes of Salalen ligands were later reported to catalyze various transformations including asymmetric oxidations, and epoxide-CO2 polymerization, but were never employed in olefin polymerization catalysis.
Exemplary additional publications include the following: Saito and Katsuki, Angew. Chem. Int. Ed., 2005, 44, 4600-4602; Shitama and Katsuki, Angew. Chem. Int. Ed., 2008, 47, 2450-2453; Yamaguchi et al., Angew. Chem. Int. Ed., 2007, 46, 4729-4731; Condo et al., Angew. Chem. Int. Ed., 2008, 47, 10195-10198; Suyama et al., Angew. Chem. Int. Ed., 2010, 49, 797-799; Berkessel et al., Adv. Synth. Catal., 2007, 349, 2385-2391; Berkessel et al., Adv. Synth. Catal., 2008, 350, 1287-1294; Matsumoto et al., Chem. Aaian J., 2008, 3, 351-358; Matsumoto et al., Chem. Comm., 2007, 3619-3627; Fujita et al, Chem. Lett., 2007, 36(9), 1092-1093; Takaki et al., Chem. Lett., 2008, 37(5), 502-503; Eno et al., Chem. Lett., 2008, 37(6), 632-633; Du et al., Inorg. Chim. Acta, 2008, 361, 3184-3192; Zeigler et al., Inorg. Chem., 2008, 48, 11290-11296; Kol et al., Inorg. Chem. Comm., 2004, 7, 280-282; Berkessel et al., J. Mol. Catal., 1996, 113, 321-342; Berkessel et al., J. Mol. Catal., 1997, 117, 339-346; Saito et al., J. Am. Chem. Soc., 2007, 129, 1978-1986; Xiong et al., Terahedon: Assymetry, 2010, 21, 374-378; Nakano et al., Macromolecules, 2009, 42, 6972-6980; and U.S. patent application having Publication Nos. 2009/0099381 and 2010/00081808.